MDL-31560 Messages: Added support for noreply and support users

This is used my messaging system for sending message from
noreply or support user. message_send api will now use
core_user class to get noreply or support user and then
send message depending on the requirement.
MDL-31560 Messages: Deprecated generate_email_supportuser

generate_email_supportuser was not creating user which can
be used by messaging system. core_user::get_support_user()
should be used to get support_user record.